Higher peak data rates and a higher cell throughput will be provided and also a wider bandwidth will be required in a future mobile communication system, e.g, a Beyond Third Generation (B3G) system or a Long Term Evolution-Advanced (LTE-A) system; and there are few existing unallocated bandwidths below 2 GHz, and a part or all of the bandwidth required for a B3G system has to be deployed in a higher frequency band, e.g., above 3 GHz. The higher the frequency band is, the faster the attenuation in radio wave propagation will be and the shorter a transmission distance will be. Thus a large number of base stations will be required to ensure coverage continuity across the same coverage area, and this will inevitably result in an increased network deployment cost due to the costly base stations. In order to address the problems of the network deployment cost and of coverage, various manufactures and standardization organizations come to work on the introduction of a relay into a cellular system for the purpose of coverage improvement.
FIG. 1 is the network architecture of an LTE-A system with deployed Relay Node (RN), where the RN accesses to a core network via a donor cell served by a Donor Evolved Node B (DeNB) and has no direct wired interface to the core network, and each RN can manage one or more cells. In this architecture, an interface between a UE and the RN reuses legacy Uu interface, and an interface between the RN and the DeNB is called the Un interface.
The RN plays two roles in the architecture illustrated in FIG. 1:
Firstly the RN plays the role of a User Equipment (UE), and the startup procedure of the RN is similar as the normal UE attach procedure. The RN has its own Serving Gateway (SGW) and Packet data network Gateway (PGW), and the RN also has its own control node, i.e., Mobility Management Entity (MME); and then
Secondly the RN plays the role of an eNB for a UE accessing to the RN, and at this time, downlink user data of the UE are transmitted from the SGW/PGW of the UE to the RN, and then transmitted from the RN to the UE via the Uu interface.
The startup process of RN consists of the following two phases:
In Phase I, the RN attaches to EPS network as a UE and then retrieves initial configuration parameters, including Donor eNB (DeNB) cells list, from the OAM system, and then the RN detaches from the network. In this phase, the eNB selects a MME for the RN, and the MME selects SGW and PGW for the RN as a normal UE.
In Phase II, the RN selects an accessible DeNB from the list of DeNB cells and establishes an RRC connection with the DeNB; the DeNB selects an appropriate MME for the RN, and the MME selects gateway nodes for the RN; the DeNB further establishes a default bearer and several required dedicated bearers for the RN, and then the OAM system downloads configuration information to the RN and completes RN configuration; and the RN can operate normally as an eNB after establishing necessary S1and X2 interfaces with the DeNB.
In the Phase I, the eNB and the MME accessed by the RN can be legacy apparatuses, i.e., apparatuses which do not support an RN. In the Phase II, the eNB and the MME accessed by the RN must support RN functionality.
In the prior art, the UE in idle state initiates an RRC connection setup procedure when the upper layer of the UE requests to establish an RRC connection with an eNB. Specifically the UE selects an appropriate cell and sends to an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) an RRC connection request message in which the UE notifies the E-UTRAN of a establishment cause including a signaling connection initiated by the UE, a data transmission connection initiated by the UE, an emergent call, a access with high-priority, an access of called UE, etc. There are also three reserved cause values.
The E-UTRAN in FIG. 3 refers to an access network node, particularly an eNB, an RN, a Home Evolved Node B (HeNB), a DeNB and other base stations. Non Access Stratum (NAS) behaviors to trigger the RRC connection setup procedure include Attach Request, Service Request, Tracking Area Update (TAU) and Detach Request. The UE embeds the NAS message in RRC Connection Setup Complete message transmitted to the E-UTRAN.
The E-UTRAN selects an MME for the UE upon reception of the RRC Connection Setup Complete message from the UE, includes the NAS message, carried in the RRC Connection Setup Complete message, in the initial UE message and then transmits the initial UE message to the MME via an S1 interface as illustrated in FIG. 4. The E-UTRAN will put the RRC establishment cause, obtained from the RRC connection request message, in the initial UE message and then transmit to the MME.
The UE carries an identity in the Attach Request message upon attachment. If the UE has no valid Globally Unique Temporary Identity (GUTI), the UE uses an International Mobile Subscriber Identity (IMSI) as its own identity. If the UE still has a valid GUTI upon attachment, then the UE uses the GUTI as its own identity. The GUTI is allocated by the MME to the UE in place of the permanent identity IMSI of the UE to prevent a user identity from being leaked due to the frequent use of the IMSI. The MME can reallocate a GUTI to the UE when the UE is attached or updated in location update or when necessary otherwise.
The MME authenticates the UE upon reception of the attach request from the UE and obtains subscription data of the UE from a Home Subscriber Server after a successful authentication and then selects SGW and PGW for the UE. An existing method for selecting a PGW can be based on a default Access Point Name (APN) in the subscription data or an APN requested by the UE in the NAS message. A principle of SGW selection is primarily to select an SGW at a close distance and with a low load according to the geographical location where the UE currently resides.
The inventors have found the following technical problems in the prior art during making of the invention:
A solution is absent to how to select SGW/PGW by MME for the RN in the phase I and the phase II of the existing RN startup procedure. If the eNB selected in the phase I by the RN is a legacy eNB which doesn't support RN functionality, but the MME supports RN functionality, then the current serving eNB will return error information if the MME selects the current serving eNB as gateway nodes of the RN, so that the RN has to reselect another eNB until the RN selects a suitable DeNB, and only then the RN can finish the startup procedure successfully, which will unpredictably increase a period of RN startup procedure. If the MME selects legacy SGW/PGW as gateway nodes of the RN in the phase II as in the prior art, then the RN will repeat constantly the process of the phase I and can not enter the normal operation.